Solar cells operate on the principle of conversion of light to electrical energy. Cadmium sulfide solar cells comprise a base of cadmium sulfide usually deposited on a suitable metal or metal coated substrate. A barrier layer is formed on the cadmium sulfide, and a metallic grid is then applied on top of the barrier layer. The barrier layer in a cadmium sulfide cell is usually a cuprous sulfide material. The barrier layer can act as a P-type semiconductor. It is integrally mated to cadmium sulfide, which can act as an N-type semiconductor material. The incidence of light on this combination of materials produces power. By connecting appropriate lead wires to the grid on the cuprous sulfide barrier layer, and to the metal substrate in contact with the cadmium sulfide, a circuit can be formed for the flow of electrical power. Such cells are well known in the art, and are described in U.S. Pat. Nos. 3,480,473; 3,975,211; and 4,167,805.
A wide variety of problems have been associated with production of the CdS-Cu.sub.2 S solar cell. It is thought, that when the crystals of cadmium sulfide are formed, they tend to drive to the top surface small amounts of impurities which cannot be accommodated by the lattice structure of the crystals. If a barrier layer is then found in proximity to the impurities, detrimental effects may result. Tanos, in U.S. Pat. No. 3,480,473, solved this problem with a H.sub.2 SO.sub.4 or HCl etch of the cadmium sulfide, to remove the impurities prior to formation of the barrier layer. This etch also improved light collection properties of the cadmium sulfide film.
Castel, in U.S. Pat. No. 4,167,805, recognized problems of copper migration in copper gridded CdS-Cu.sub.2 S cells. After the Cu.sub.2 S is formed on the base CdS, it is necessary to "heat treat" the cell, for a maximum of 10 minutes at up to 260.degree. C., to complete the formation of the CdS-Cu.sub.2 S heterojunction. Metallic copper, from a copper electrode grid attached prior to "heat treatment", and from the Cu.sub.2 S layer, readily diffuses into the cell. This lowers the copper content of the Cu.sub.2 S barrier and produces undesirable degration in cell performance. The higher the "heat treatment" temperature and the longer the duration of "heat treatment", the more degradation. Castel solved this problem, in part, by forming a Cu.sub.2.00 S layer and then a Cu.sub.2.00-x S layer, where x may be from about 0.05 to 0.20. Upon "heat treatment", diffusion of copper causes the copper deficient lattice of the Cu.sub.2.00-x S to fill and form the desired high efficiency Cu.sub.2.00 S material. This dual cuprous sulfide formation is a very complicated process, however.
What is needed is a simple and inexpensive method of using and isolating metallic mesh grids in CdS-Cu.sub.2 S solar cells. The method should be mechanically straightforward and repeatable, so that equipment costs, manpower costs and rejection rates are minimized.